In a transmission of a hybrid drive-train of a motor vehicle, to ensure the supply of oil when the internal combustion engine is switched off, for example when the vehicle is at rest or during purely electric-powered driving, an electric auxiliary pump is needed. The auxiliary pump ensures that gearshifts can be carried out during driving under electric power and that the transmission is cooled. As a rule the auxiliary pump is only operated when the main oil pump powered by the internal combustion engine or by some other drive aggregate can no longer ensure the necessary volume flow or oil pressure.
To obtain trouble-free operation of the transmission, it must be ensured that when there is a need to operate the electric auxiliary pump, that pump can provide the necessary volume flow and oil pressure both for the primary circuit, comprising the clutch valve controls, and for the secondary circuit, comprising the cooling and lubrication systems for the transmission components of the transmission.
In the first place the electrically operated auxiliary oil pump supplies the primary circuit with oil; to supply the secondary circuit with oil the operating pressure of the auxiliary oil pump must be increased to above the minimum system pressure, which is as a rule 5.5 bar, so that the primary circuit becomes saturated and the excess oil quantity produced in this way can then flow into the secondary circuit. This results on the one hand in a high electric power uptake by the auxiliary oil pump and on the other hand in severe stressing of the auxiliary oil pump, which can lead to damage thereof since the auxiliary oil pump is not designed to endure prolonged delivery of a pressure in excess of 5.5 bar. Disadvantageously, to design the auxiliary oil pump to withstand this results in higher costs; furthermore, owing to the restricted fitting space available the fitting of an auxiliary oil pump designed to endure a delivery pressure higher than 5.5 bar is often problematic.
From DE 10 2008 040 667 A1 by the present applicant a hybrid system of a transmission device is known, which comprises a main transmission pump that can be driven by a torque transmitted by way of the transmission device and which also has an auxiliary oil pump that can be driven by an electric machine, by means of which a primary pressure circuit and a secondary circuit can be provided with hybrid fluid in an operating condition dependent manner.
In this case a pressure side of the main transmission pump and a pressure side of the auxiliary pump are connected to the primary pressure circuit upstream from a pressure-limiting valve provided in order to set a main pressure of the primary pressure circuit, the pressure-limiting valve being arranged between the pressure sides of the main transmission pump and auxiliary pump and the secondary pressure circuit, while the pressure side of the auxiliary pump can be brought into functional connection with the secondary pressure circuit by way of a hybrid line that bypasses the pressure-limiting valve and can be blocked in the direction toward the primary pressure circuit and the secondary pressure circuit.